Field of the Invention
The present invention generally relates to a brake apparatus for railway vehicles and, more particularly, to a service graduating valve and service slide valve of a service portion of a brake control valve.
Description of Related Art
China Railways have a standardized pneumatic air brake valve, called “Type 120-1”, which is similar in overall operation to American Association of Railroads (AAR) pneumatic air brake valves (such as the WABCO ABD), with a pipe bracket, service valve portion, and emergency valve portion. Testing performed on the Type 120-1 valves reveal that during specific maintained brake pipe reductions, the Accelerated Application Valve function (AAV) can find a position in which it will ‘stick on’ and, thus, not be able to properly shut down. Therefore, the Type 120-1 service valve portion will continue to exhaust brake pipe pressure locally at each freight car where this condition exists. When this occurs, this causes a further decrease in brake pipe pressure in the train and greatly increases the risks of undesired brake releases since the train's locomotive contains the pressure maintaining feature. Depending upon the number of service control valves that exhibit this undesirable function in a train consist, an increased risk of subjecting the train to an undesirable release condition exists. The AAV function helps to drive the brake pipe exhaust signal locally at each control valve, which therefore provides faster brake cylinder development during service brake applications. This function results in better overall train stopping ability and helps to stop longer and heavier freight trains in a more efficient manner.
Particularly, train tests performed on fifty 50-foot OEM 120-1 type control valve portions revealed that after a certain maintained brake pipe reduction, the AAV function would remain ‘stuck on’ and not shut off properly, even with the locomotive pressure maintaining feature. Train tests performed revealed as many as 40% of the cars having a ‘stuck AAV’ condition as observed during one particular test.
The effects of having AAV's ‘stuck on’ in a train will result in a further decrease in brake pipe pressure from its maintaining pressure state, which can, therefore, significantly increase the risk of undesired brake releases. Since the Chinese 120-1 type service valve's Service Accelerated Release (SAR) function is piloted by brake cylinder exhaust pressure, each time the control valve goes to the release position, the SAR function allows accelerated release reservoir pressure to dump back into the brake pipe locally on each car. When this function occurs, a significant increase in the risk of releasing the brakes on additional cars, or the entire train, exists because brake pipe pressure locally in the train is being increased. Train tests performed on fifty 50-foot car Chinese 120-1 type control valves did not reveal an undesired brake release when the AAV ‘stuck on’ condition was observed; however, the reason for this was due to the short train make-up of only 50 cars.
However, on longer trains, such as 7,500 feet or longer, there will be an increased risk of undesired brake releases if the AAV function ‘sticks on’, primarily due to the additional number of valves in the train possessing the AAV ‘stuck on’ issue. The greater number of service valves in a train having the AAV ‘stuck on’ issue will decrease brake pipe pressure further below from its ‘maintaining’ pressure state. Thus, when all the ‘suspect’ service valve portions in the train, which have an AAV ‘stuck on’ issue present, move from their service position to their boost position, the AAV function will be nullified. However, when this occurs, the pressure maintaining wave in the train is going to increase brake pipe back to its maintaining state, which, in turn, provides the proper dynamics to drive undesired brake releases. Any undesired brake release in a train consist, where potentially the entire train could possibly be undesirably released, is a major train handling safety issue that must be avoided at all costs.